Interaction,Mechanism And Properties Of Cellulose Dissolved In Alkaline Aqueous Solution

Facing the threats of the depletion of non-renewable energy and environmental pollution caused by petroleum-based polymers,the trend of science and technology is tending towards environmentally friendly materials,renewable resources and energy,as well as sustainable techniques and processes.Cellulose,as the most abundant natural polymer on earth,has attracted a number of attentions,due to its renewability,wide availability,low-cost,biocompatibility and biodegradability,etc.The stiff cellulose molecular chain and their close packing through numerous hydrogen bonds have made the dissolution of cellulose to be a difficult process.Our group have developed cellulose solvents based on alkali/urea aqueous system,providing a novel pathway for the utilization of cellulose via a "green" process.This thesis focuses on the interaction,mechanism and properties of cellulose dissolved in alkaline aqueous solution.By a combination of nuclear magnetic resonance(NMR)spectroscopy,molecular dynamic(MD)simulation,Fourier transform infrared spectroscopy(FT-IR),dynamic and static light scattering(DLS/SLS),differential scanning calorimetry(DSC),atomic force microscopy(AFM)and many other techniques,the interactions between the components in the solution and the mechanism of dissolving the macromolecules are studied.Furthermore,the molecular parameters and molecular weight of cellulose,as well as its Mark-Houwink equation are determined.Moreover,from the perspective of diversity and universality,more kinds of alkaline aqueous solvents and more soluble macromolecules in the systems are investigated.The main content of the thesis include:(1)Breaking through the traditional 1H diffusometry,interactions of cations with cellulose were investigated with 7Li and 23Na PFG-SE NMR.Diffusion coefficient of Li+ decreased more than Na+ with the addition of cellulose,indicating the stronger binding of LiOH with the macromolecule.Therefore,a new,facile,accurate and repeatable method to characterize ion/polymer interaction was established.(2)The influence of cation on the cellulose dissolution in alkali/urea by a cooling process was investigated with a combination of MD simulation and experiments,including DSC,NMR diffusometry(PFG-SE NMR)and FTIR.The results of DSC proved that the dissolution of cellulose in both solvents was typically enthalpy-driven,starting at a temperature above 0? and completing at low temperature(-5? for LiOH/urea and-20? for NaOH/urea),indicating the necessity of low temperature for the cellulose dissolution.Molecular dynamic(MD)simulation revealed that the electrostatic force between OH-and cellulose dominated the dissolution among their inter-molecular interactions.In our findings,Li+ could penetrate closer to cellulose due to the smaller radius,and displayed stronger electrostatic interaction with the biomacromolecule than Na+,thus possessed a greater "stabilizing" effect on the OH-/cellulose interaction,leading to a more powerful dissolving capacity.PFG-SE NMR demonstrated a more significant binding fraction of Li+ than Na+ to cellulose,which was consistent with MD.These results indicated that the direct interactions existed between the cations and cellulose,and Li+ exhibited stronger interaction with cellulose,leading to stronger dissolving power.Furthermore,the FITR results indicated that the water structure in LiOH/urea displayed a greater stabilization effect on the hydrophobic site of cellulose molecules,due to the larger proportion of the donor-acceptor water mode in LiOH/urea.(3)The characterizaion of weak interactions between urea and cellulose in the alkaline aqueous solution,which were usually ignored or underestimated were accomplished and confirmed in-situ,for the first time,with the new insights from PFG-SE NMR,FT-IR and solvatochromic methods,etc.The NMR results indicated the binding of urea with cellulose in the solution,demonstrating the existence of the weak interactions between them.Subsequently,solvatochromic methods revealed that urea hardly affected the hydrogen bonding donor(HBD acidity)and hydrogen bonding acceptor(HBA basicity)properties of the solvent,but was related to its dipolarity and polarizability,indicating that the dispersion forces existed therein,but not likely the hydrogen bonding,which was also supported by the FT-IR.Furthermore,the impact of weak interactions between urea and cellulose was demonstrated to facilitate the dissolving process.The fine dispersion and good stability of cellulose in the solution were maintained by mitigating the effect of the hydrophobic portions from all the dilute,semi-dilute to the concentrated regimes,supported by the results of dynamic light scattering(DLS),rheology and NMR,etc.Therefore,the transmittance and mechanical properties of the regenerated cellulose materials prepared from the cellulose solution in alkali/urea aqueous system were enhanced,compared with that in only alkali system.(4)The chain conformation and molecular weight of cellulose in LiOH/urea were investigated by DLS/SLS,AFM,TEM and so on.The structure factor,persistent length and other molecular parameters were determined to prove the stiff conformation of cellulose in the solution,which was also supported by AFM images.Moreover,the Mark-Houwink equation of cellulose was also established.(5)A new kind of solvent based on NaOH and zinc nitrate was developed to dissolve cellulose at low temperature.NMR proved that the dissolving was a physical process.The good solubility and stability of cellulose in the solution were also confirmed.Furthermore,novel cellulose/ZnO microspheres were fabricated based on the solution.SEM,XRD and FT-IR were used to characterize the microspheres,which exhibited porous structures embedded with ZnO particles.Moreover,their properties of antibacteria and dye degradation were evaluated,which showed potential application in the fields of environmental chemistry.(6)For the first time,agarose derived from red algae of seaweeds was dissolved successfully in alkaline aqueous solution via freezing/thawing to form a transparent solution,in contrast to the ordinary heating method.The dissolution was a physical process,which was investigated by 13C NMR.The results of dynamic light scattering and atomic force microscopy demonstrated that agarose existed as extended chains in the solution and easily aggregated in parallel to form nanofibers.The viscosity and rheology measurements indicated that the agarose solution exhibited higher stability at room temperature than that traditionally dissolved in hot water.Moreover,the regenerated agarose hydrogels were fabricated directly from the agarose solution,showing more homogenous structure,enhanced thermal stability and mechanical properties than that prepared by hot water.Importantly,the compression fracture stress of the agarose hydrogels was 3.7 times of that prepared by the traditional hot water method,as a result of the reinforcement by the nanofibrous microstructures,leading to greater applicability.Furthermore,the agarose hydrogels displayed excellent biocompatibility,showing potential applications in the fields of biomaterials.